Boris Johnson is on course to be re-elected as London mayor on 3 May, according to the latest ComRes poll giving the Conservative candidate a clear lead over his Labour rival, Ken Livingstone.

The poll shows Johnson enjoying an eight-point lead, just three days after a YouGov poll put the two candidates virtually neck and neck. The latest snapshot will make sober reading for Livingstone as he seeks to woo voters in the last week of his campaign to oust Johnson on May 3.

The latest survey was published on a day that Labour MP Tom Watson told LBC 97.3 listeners that "those Lab voters thinking of going to vote for Boris, hold your nose, vote for Ken" as he warned that a win for Johnson would be interpreted by David Cameron as a "vote of confidence in him".

In a clear sign that Johnson appears immune to the troubles of his Tory colleagues in Westminster, the incumbent mayor appears to be forging ahead of Livingstone in his bid to secure a second term at a time when the Conservative party nationally is trailing Labour.

The ComRes poll of 1,024 Londoners found 45% of voters would give Johnson their first preference vote, compared with Livingstone on 36%. When other candidates are stripped out in the second round of the preference voting system, Johnson has an eight percentage point lead (54%-46%).

The poll, which was conducted before the latest growth figures showed Britain plunging into its first double-dip recession in almost four decades, shows respondents believed Johnson was best placed (40%) to improve London's economy than his Labour rival (24%).

The survey shows voters trust Johnson more (29% to 20%), and has the most attractive policies (32%to 29%). This is despite the fact that the top priority for Londoners (63%) is keeping down the costs of public transport – a flagship promise from Livingstone who has pledged to cut fares by 7%.

Johnson was viewed as the most honest about what either candidate would do in office (27% to 22%) and has been the most transparent about his tax affairs (29% to 15%). Respondents also felt Livingstone had had the most disappointing campaign (26%to 12%).

Other key priorities listed by respondents was ensuring there is enough affordable housing in the capital (47%) and restoring confidence in the police (38%).

Fourth on the list of priorities (32%) is the idea of London keeping more of the tax generated in London, a proposal Johnson has highlighted in his re-election manifesto.

With just a week to go until voting, Watson told LBC presenter Nick Ferrari that a section of voters who say they would vote Labour in a general election intend to vote for Johnson in the mayoral election, claiming "they either like him – he's a nice, charismatic guy – or they don't like Ken".

The YouGov poll published on Monday showed 11% of Labour supporters surveyed say they intend to vote for Johnson.

Watson, deputy chair of the Labour party, said Labour's task in the last week of campaigning "is to convince those people that if you vote for Boris, then you're also giving a vote of confidence to David Cameron and the economy and if you vote for Ken, then you're sending a message to David Cameron."

He added: "David Cameron will interpret Boris Johnson's success as a vote of confidence in him, I promise you that … For once, I'm being totally candid with you. I'm saying to you, those Labour voters that are thinking of going to vote for Boris Johnson, hold your nose, vote for Ken, because that's the way that you will help Labour."

While the ComRes poll shows Johnson making strides over his main rival, the survey also highlights a boost for minority parties, with Green mayoral candidate Jenny Jones in third place (6%) on weighted turnout for first preference votes, ahead of Liberal Democrat candidate Brian Paddick (5%).

Independent candidate Siobhan Benita is in fifth place on 3%, ahead of Ukip's Lawrence Webb (2%).

Jones, who is also standing in the London assembly elections was delighted to be in third position after overtaking her Lib Dem rival, said: "Polling is of course volatile, unpredictable and often unreliable, but I'm pleased this election gives the Green party an opportunity to provide an alternative vision to the Punch and Judy politics that have dominated the race – and British politics in general."

Benita, who has been battling for airtime as the only independent candidate in the race, said: "I'm delighted I've now got such a large percentage of 2nd preference votes, despite the fact that my campaign has been hindered by me being barred from the big TV debates and from having an election broadcast. There's a real momentum building around me as Londoners realise they do have a sensible, credible alternative to the tired old candidates from the main political parties."